In article , says...
Cupla people have mentioned programmable/vector drive stuff. I would
imagine this "costs extra". Is it worth it?

Probably not for your grinder. True vector drive requires encoder
feedback and is desirable when you require full torque at very low RPMs,
or accurate speed control. Sensorless or flux vector control is a
compromise between V/Hz drives and true vector drives and is included in
many low cost programmable drives these days.

The programmable drives will have literally hundreds of settable
parameters that you don't need if all you want to do is turn your
grinder on and off with a switch and change the speed with a pot.

Is there anything ito of older/cheaper VFDs that I should avoid?

Some of them generate more electrical noise than newer or higher end
drives. I'm a big fan of Yaskawa drives. Both the older GPD series
(which you may see with a Magnetek label) and the newer V7/F7/G7 drives
are good solid drives and are well documented. Mitsubishis are also
generally very good, but the documentation can be cryptic.

Ned Simmons